Turbulent density fluctuations in the solar wind

Abstract

Measurements of density, electric field, magnetic field, in the solar wind have revealed fluctuations in these quantities spanning a large range of scales, indicative of turbulence. The nature of turbulence in the solar wind has been the subject of intense research as it plays an essential role in several aspects of plasma behavior such as, solar wind acceleration and heating of the extended solar corona and solar wind. While considerable progress has been made, the nature of turbulent dissipation, especially in the extended solar corona, and the role of density turbulence therein remains a significant unsolved problem. This thesis is concerned with the nature of density turbulence in the extended solar corona, especially near the inner/dissipation scale. Electromagnetic waves traversing the solar wind experience scattering due to turbulent density fluctuations, which leads to a wide variety of observed phenomena such as intensity scintillations, angular broadening, pulse smearing, etc. These observations provide useful constraints on the quantities characterizing density turbulence. Chapter 2 provides an overview of the phenomenon of angular broadening. Treatments of the radio scattering due to density turbulence in the solar wind typically employ asymptotic approximations to the phase structure function. In chapter 3 we use a general structure function (GSF) that straddles the asymptotic limits and quantify the relative error introduced by the approximations. We show that the regimes where GSF predictions are accurate than those of its asymptotic approximations is not only of practical relevance, but are where inner scale effects influence the estimate of the scatter-broadening. Thus we propose that GSF should henceforth be usedfor scatter broadening calculations and estimates of quantities characterizing density turbulence in the solar corona and solar wind. In the next part of this thesis we use measurements of density turbulence in the solar wind from observations of radio wave scattering and interplanetary scintillations. Density fluctuations are inferred using the GSF for radio scattering data and existing analysis methods for IPS. Assuming that the density fluctuations below proton scales are due to kinetic Alfvèn waves, we constrain the rate at which the extended solar wind is heated due to turbulent dissipation. These results, elaborated in chapter 4, provide the first estimates of the solar wind heating rate all the way from the Sun to the Earth.